1. Field of the Invention
The present invention relates to an imaging lens which forms an optical image of a subject on an imaging element such as a CCD (Charge-Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), and relates to a fixed-focus imaging lens suitable for a camera module for converting the optical image formed by the imaging lens into an image signal, and a camera-equipped portable cellular phone or an information portable terminal (PDA: Personal Digital Assistance), which is equipped with the imaging lens to capture an image.
2. Description of Related Art
Significant progress has recently been made toward miniaturization and an increased number of pixels of an imaging element, such as a CCD or a CMOS. To this end, a compact, high-performance imaging device main unit and a lens mounted on the main unit are sought. In order to address an imaging element having a larger number of pixels, telecentricity; that is, a property for rendering an angle of incidence of a principal ray to an imaging element closely parallel to an optical axis (i.e., the angle of incidence to an imaging surface close to a value of zero with respect to the normal to the imaging surface), has been sought. A lens of a three-lens configuration has hitherto been developed as a compact imaging lens (see JP-A-2003-322792, JP-A-2005-352317, JP-A-2005-17440, JP-A-2005-292235, JP-A-2005-345919, JP-A-2004-4566, JP-A-2004-226487, JP-A-2004-240063 and JP-A-2006-47858). JP-A-2003-322792 and JP-A-2005-352317 and JP-A-2005-17440 (a first example) describe an imaging lens of a so-called middle aperture configuration in which an aperture diaphragm is interposed between a first lens and a second lens. However, in the case of the middle aperture configuration, when an attempt is made to shorten the entire length of a lens system, the angle of incidence of the principal ray to the imaging surface becomes greater, which in turn worsens the telecentricity. Accordingly, a conceivable layout is to place the aperture diaphragm at a position closest to an object side in order to ensure the telecentricity. JP-A-2005-17440 (a second embodiment) and JP-A-2005-292235, JP-A-2005-345919, JP-A-2004-4566, JP-A-2004-226487, JP-A-2004-240063 and JP-A-2006-47858 describe an imaging lens of a three-lens configuration in which an aperture diaphragm is placed at a position closest to the object side.
Development of a lens exhibiting higher optical performance is expected in step with an increase in the number of pixels of the imaging element. Development of an imaging lens whose curvature of field and chromatic aberration are properly corrected while being miniaturized has been desired. When the aperture diaphragm is placed at the position closest to the object side, variations in sensitivity due to manufacture (i.e., the degree of change in optical performance ascribable to variations in manufacture) tend to increase, and mass-production suitability of the imaging lens tends to become worse. Accordingly, development of an imaging lens whose telecentricity is ensured by placing an aperture diaphragm at a position closest to an object side and whose change-in-manufacture sensitivity is reduced has been desired.